Infrared Spectra: Primary Clues
Information on characteristic peaks associated with specific
functional groups is tabulated below
and illustrated on the linked page for each
group. [opens in same frame, use "back" to return here]
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| Class of Compound Functional Group |
characteristic bands due to |
~ peak location, cm-1 |
peak description & comments |
|
Alkanes and alkyl groups |
C-H stretch
CH3 'umbrella' CH2 scissor |
to right of
3000
1375 1450 |
med-strong, med-broad
med-weak, sharp usually med, sharp |
| Alkenes | =C-H stretch
C=C stretch* =C-H oop bend |
to left of
3000
1600-1675* 650-1000 |
med, sharpish
med-weak, sharp position can be correlated to pattern of substitution |
| Aromatics | =C-H stretch
ring C=C stretch* overtones =C-H oop bend |
to left of
3000 1450-1600 1667-2000 690-900 |
med, sharpish
sharp, weak very weak, diagnostic for substitution pattern |
| Alkynes | C≡C
stretch*
≡C-H stretch |
~2150*
~3300 |
weak, sharp
sharp? |
| Nitriles | C≡N stretch | 2300 | medium, sharp |
| Alcohols | O-H stretch
C-O stretch |
3200-3600
1000-1200 |
medium, broad
strong |
| Ethers | C-O stretch | 1000-1200 | strong |
| Amines | N-H stretch
N-H scissor C-N stretch |
3300-3500
1560-1640 1000-1350 |
broad; split for 1o,
not for 2o
broad med-weak, easy to miss |
| Aldehydes | C=O stretch*
CHO C-H stretch |
1725*
2750 and 2850 |
strong, med-width
characteristic "sawteeth" on right of aliphatic C-H stretch region |
| Ketones | C=O stretch* § | base value 1715*§ | the C=O stretch is generally the strongest peak in any IR where it is present |
| Carboxylic Acids | O-H stretch
C=O stretch* C-O stretch |
2500-3500
|
broad, may
show "beard" of overshadowed C-H stretches strong medium, often hard to distinguish from other peaks in "fingerprint region" |
| Esters | C=O stretch* § C-O stretch |
~1735 1000-1300 [2] |
strong fairly strong; |
| Amides | C=O stretch*
§ C-N stretch N-H stretch N-H bend |
1640-1670
1000-1350 3100-3500 1550-1640 |
strong often lost in "traffic" broad medium |
| Anhydrides |
C=O stretch*
§ C-O stretch |
1740-1790
and 1800-1850 |
strong [as usual] C-O bond has more sp2 character than normal |
| 1200-1300 | |||
| Nitro | N=O/N-O stretch | 1300-1390
and 1500-1600 |
both strong |
| Halides | C-F stretch
C-Cl stretch C-Br stretch C-I stretch |
960-1350
500-800 to right of 667 to right of 667 |
strong, broad |
| * conjugation moves these peaks to the right [lower frequency] | |||
| § increased ring strain when C=O is cyclic moves these peaks to the left [higher frequency] | |||
| See a graphic summary of where the peaks are | |||
| Test your skill in identifying some of the most characteristic IR peaks | |||
| © Ronald W. Rinehart, 2002, 2003, 2004 "Actual results may vary." | |||